Microalgae is a source of biofuels that offers an alternative to the use of conventional fossil fuels. Microalgae are primitive plants that grow in aqueous suspensions. Microalgae include single-celled photosynthetic microorganisms that utilize energy from sunlight to combine water and carbon dioxide to produce biomass that can contain significant quantities of lipids similar to that found in vegetable oils. The lipid material can be used in the production of biofuels through the chemical process known as transesterification. Reproduction of microalgae is primarily through asexual cell division, however, under certain growth conditions, sexual reproduction can occur. There are a number of groups of algae with thousands of known species. Some of the major classes studied for generation of algal biofuels are: the Diatoms; Green Algae; Golden-Brown Algae; Prymnesiophytes and the Eustigmatophytes.
A significant body of research demonstrates that photosynthetic microalgae can be propagated in open raceway ponds or in photosynthetic bioreactors and harvested for biofuel production. Microalgae have also been cultivated in the dark by fermentation to produce biofuel. Microalgae form storage lipids primarily in the form of neutral triglycerides that can be used to produce biodiesel through transesterification using alcohol and an acid or base as a catalyst.
The successful commercialization of microalgae as a biofuel source depends on a number of factors. The most significant are increased lipid (oil) yield and/or biomass growth enhancement and cost efficient harvesting methods.
A number of studies demonstrate that nutrient limitation such as nitrogen and/or silicon deprivation can induce algae to increase their lipid content. The downside to nutrient deprivation is a cessation of cell division and cell growth and the corresponding limitation of biomass such that the net yield on lipid production is not significant.
Further, the cost of harvesting microalgae from an aqueous environment is a limitation to successful commercialization of microalgae as a biofuel. One method for harvesting algae uses a microstrainer to screen out the algae. In this method, the water containing the suspended microalgae from an open raceway pond flows over a rotating or fixed microstrainer to collect the algae while the water passes through. The microalgae that is collected on the microstrainer is swept off by a moving scraper or backwashed and collected for centrifugation to concentrate the algae for extraction of the lipids.
Another method of harvesting microalgae involves allowing microalgae cultures to settle under the influence of polymers and FeCl3. The high doses used and the high cost of flocculants (2-6 g/kg for organic flocculants and 15-200 g/kg for FeCl3) combined with the high cost of cross-flow filtration used for harvesting is commercially undesirable. Other variants of the method involve centrifugation of the settled algae, which is also impractical and expensive.
Another method for harvesting microalgae is chemical flocculation followed by dissolved air flotation. The lipid contents were low for the flocculated algae harvested by dissolved air flotation compared to sedimented algae harvested by centrifugation.
Collection of algae by filtration over a bed of sand is another method for harvesting microalgae, but is not practical due to the difficulty and cost associated with removing the collected algae from the sand bed.
In view of the foregoing problems with conventional methods for growing and harvesting microalgae, it would be desirable to provide a method to enhance the growth rate of algae and/or increase the lipid content of algae without inhibiting cell division and that further allows effective harvesting.